1. Field of the Invention
The present invention relates generally to digital cellular mobile radio communication systems. The present invention relates more specifically to a method and system for effecting an improved mobile assisted handoff operation as the mobile station moves from one cell to another within a cellular communication network.
2. Description of the Related Art
In cellular mobile communication systems where a mobile station (MS) may be constantly moving, it is essential that the radio communications link with the mobile station be maintained when the station moves from one cell served by one base station (BS) to a second cell served by a different base station. The process of handing over the communications link from one base station to another, in a manner that is transparent to the user, is known as handoff.
The most commonly used criteria to determine the need for a handoff is received signal strength (RSS). In first generation cellular systems such as the Advanced Mobile Phone System (AMPS), the serving base station performs a received signal strength measurement on the signal received from the mobile station and uses this information to make a handoff decision. In such a system and method, there is no mobile station involvement in the measurement or decision making process.
Second generation digital cellular communication systems such as the IS-136 and GSM systems have introduced mobile assisted handoff (MAHO) where the mobile station assists the base station in the handoff decision making process by itself performing received signal strength and bit error rate (BER) measurements.
In the IS-136 system, the mobile station performs received signal strength and bit error rate measurements on the channel it is currently operating on and only received signal strength measurements on a list of candidate channels supplied to it by the base station. The RSS measurements on candidate channels are performed during the idle period between designated time slots in the communication link.
A technique called Digital Locate has also been developed by network infrastructure manufacturers that allows candidate base stations to perform received signal strength measurements on the mobile station in question and allows the base station to synchronize to the mobile station's transmissions to verify the identity of the mobile station.
In the GSM system, the mobile station also performs received signal strength measurements during the idle period between designated time slots. In addition, since the frame structure of GSM has one idle time frame per multi-frame, during this idle time frame the mobile station synchronizes to the broadcast control channel (BCCH) of a candidate base station and reads the base station identification code (BSIC) to verify the identity of the base station. A number of patents, including U.S. Pat. Nos. 5,042,082, 5,200,957, and 5,257,401, describe systems within which the mobile station performs received signal strength measurements on a list of target base station channels. The base station then supplies the color code information of the mobile station to the target base stations and the target base stations measure the received signal strength of the mobile station in question and synchronize to the mobile station's transmissions to read its color code.
Other patents, including U.S. Pat. No. 5,493,563, describe systems where measurements on voice channels of adjacent cells are performed. In addition, U.S. Pat. No. 5,157,661 describes a system where a communication test is performed on candidate channels during the free time of a time division multiplexed channel.
Despite the above efforts at improving the mobile assisted handoff operation, there are still serious drawbacks to the accuracy of the systems and their ability to identify and designate the most appropriate base station to receive the handoff. By involving the mobile station in the appropriate selection of a base station handoff, the accuracy improves, but remains a problem even with the kinds of measurements mentioned in the above patents.